1. Field of the Invention
The invention relates to a horizontal deflecting circuit suitable for use in a multiscan type television receiver which can receive video signals of different horizontal frequencies.
2. Description of the Related Art
Generally, in a television receiver corresponding to a plurality of different horizontal frequencies, in order to assure a necessary raster width, a power voltage which is different for every frequency is supplied to a horizontal deflecting circuit. FIG. 6 shows an example of such a horizontal deflecting circuit. In FIG. 6, reference numeral 21 denotes a variable voltage power source. A (-) terminal of the variable voltage power source 21 is connected to the ground. A (+) terminal of the power source 21 is connected to the ground via a coil 22, a horizontal deflecting coil 25, and an S-shaped correcting curve provided by capacitor 26. A switching element 23 for horizontal output is connected between the ground and a node of the coil 22 and horizontal deflecting coil 25. This node is connected to the ground via a resonance capacitor 24.
The variable voltage power source 21 supplies an electric power necessary for maintaining the resonance to the horizontal deflecting circuit through the coil 22. For a tracing period of time during which the horizontal outputting switching element 23 is turned on, a voltage of the capacitor 26 is applied across the horizontal deflecting coil 25. When a capacitance of the capacitor 26 is enough large, a saw-tooth wave current having a predetermined inclination flows in the horizontal deflecting coil 25 and a saw-tooth wave current having a predetermined inclination also similarly flows in the coil 22.
Subsequently, when the horizontal outputting switching element 23 is turned off for a retracing period of time, the current flowing in the horizontal deflecting coil 25 and coil 22 flows into the resonance capacitor 24, so that a voltage (called a retrace pulse) is generated across the resonance capacitor 24. A voltage of the retrace pulse generated across the resonance capacitor 24 reaches a peak value at time Tr/2 shown by the following equation (1). EQU Tr/2=(.pi./2).times. (L.cndot.C) (1) EQU L=L1.cndot.L2/(L1+L2)
where,
L1: inductance of the coil 22 PA1 L2: inductance of the horizontal deflecting coil 25 PA1 C: capacitance of the resonance capacitor 24 PA1 V: voltage of the variable voltage power source 21 PA1 Tt: tracing period
In equation (1), ( ) denotes the square root of the value in the parentheses.
A peak value Vcp of the retrace pulse is obtained by the following equation (2). EQU Vcp=V.cndot.(1+(.pi./2).cndot.(Tt/Tr)) (2)
where,
After that, the resonance capacitor 24 discharges the accumulated charges. When the voltage across the capacitor 24 is equal to 0, the horizontal outputting switching element 23 is again turned on and the foregoing operation is repeated. Thus, the saw-tooth wave current flows in the horizontal deflecting coil 25. In an actual circuit, by a combination of a bipolar transistor and a diode is often used as a horizontal outputting switching element 23 and, when the voltage across the resonance capacitor 24 exceeds a forward voltage of the diode, the circuit is automatically short-circuited.
The applicant of the present invention has proposed a multiscan type television receiver disclosed in JP-A-61-96875 (Japanese Patent Application No. 218863/1984). According to a horizontal deflecting circuit in the television receiver, a variable voltage power source is used. The variable voltage power source is controlled so that the product of a power voltage from the variable voltage power source and a horizontal tracing time is made constant. A serial circuit of a plurality of capacitors and a connection switch is connected in parallel with a resonance capacitor. By controlling so as to increase an electrostatic capacitance of a horizontal retrace capacitor as a frequency of a horizontal sync signal decreases and to make a high output voltage constant, it is possible to cope with a change in horizontal frequency of about twice.
However, in case of the horizontal deflecting circuit using the variable voltage power source, it is necessary that a dedicated variable voltage power source unit for the horizontal deflecting circuit is added separately from a main power supplying unit to output a predetermined voltage or the main power supplying unit itself is used as a variable voltage power source. There are consequently problems such that a construction of the power supplying unit becomes complicated, a circuit scale of the whole apparatus increases, and the costs rise.
In case of setting a duration of the retracing period constant, as shown in the equation (2), since the voltage of the retrace pulse generated for the retracing period rises as the horizontal frequency decreases, there is a possibility that unless the change in power voltage can transiently trace the frequency change, a voltage higher than the ordinary one is applied to elements constructing the circuit. There are, thus, problems such that a circuit construction to prevent the reliability from being lost is necessary and, further, the circuit becomes complicated.
Moreover, in a horizontal deflecting circuit of the conventional type such that the coil 22 provided between the horizontal deflecting coil 25 and variable voltage power source 21 is set to a transformer structure like a flyback transformer and a secondary output such as a high voltage or heater voltage which is applied to an anode of a cathode ray tube (CRT) is extracted from the secondary coil 22, by changing a power voltage that is supplied to the horizontal deflecting circuit, a peak value of the retrace pulse and a voltage for the tracing period change. Since the secondary output is formed by rectifying the retrace pulse and the voltage for the tracing period, the secondary output also fluctuates and there is a problem that a necessary constant voltage cannot be extracted.
In FIG. 7, a solid line waveform and a broken line waveform indicate retrace pulses when horizontal frequencies are different. Although a voltage according to a turn ratio with a primary coil is generated in the secondary coil of the transformer, as shown in FIG. 7, a voltage from a mean value of 0, namely, AC=0 is rectified for the tracing or retracing period and is extracted. There is, consequently, a problem that the secondary output fluctuates.